Hi, Dave Douglas, resident of PebbleCreek here…

Aside from cost, you may have heard differing reasons to keep an existing furnace when your outdoor unit (condenser) or the coil is replaced. However, the best reason to replace your furnace when you replace the outdoor unit is for optimum air flow…the older furnaces cannot provide optimum airflow for the new refrigerant (R410a).

The old refrigerant (R-22) “normal operating pressures” are 75# suction line and 270# liquid line. The new refrigerant (R410a) “normal operating pressures” are 145# suction line and 390# liquid line. Higher pressures = higher temperatures.

R-22 evaporator coils are typically designed at a 34-37 degree Fahrenheit wet bulb temperature. R410a evaporator coils are typically designed at a 39-42 degree Fahrenheit wet bulb temperature. Since the new refrigerant (R410a) evaporator coil is “a little warmer,” the air flow needs to be very accurate in order to achieve the rated efficiencies.

HVAC efficiency is measured using the Seasonal Energy Efficiency Ratio (SEER) rating, which is defined by the Air Conditioning, Heating, and Refrigeration Institute. Generally, the higher the SEER rating, the more efficient the equipment and the less energy it will consume to make your home comfortable. In other words, your electric bill!

Three things determine SEER rating when you purchase new HVAC equipment: outdoor unit, evaporator coil, AND furnace. If you change only the outdoor unit and the evaporator coil, the EPA says we can “say” that the SEER rating is 14, but we cannot verify it. The furnace can increase the SEER rating by going from a standard blower furnace to a true variable speed blower furnace.

An old furnace may still be “functioning,” but you truly will not be reaping the benefits of a new efficient system without having a new furnace to work with the new outdoor unit and coil.